The present invention relates to a redispersible synthetic resin powder comprising as a main component a resin made of an ethylenically unsaturated monomer and/or a diene monomer, such as acrylic resins, and the use of the powder. More particularly, the present invention relates to a redispersible synthetic resin powder having excellent properties, such as blocking resistance, dispersibility in redispersion, film forming property and water resistance of films formed from the powder, and being useful as an admixture for cement and mortar. The present invention also relates to an admixture for cement and mortar comprising the powder.
Emulsions of synthetic resins such as vinyl acetate resins and acrylic resins have been widely used in adhesives, coatings and the like. The emulsions are generally supplied in the form of liquid. The supply of liquid emulsions is disadvantageous in that the transportation is expensive, emulsions may freeze in winter or change its quality with the lapse of time and treatment of waste water may be required after the use of emulsions.
In view of these disadvantages, recently, emulsions are dried, and powders are supplied and used in various fields. Upon the use, the powders are dispersed into water again. Therefore, the powders are required to have a good redispersibility into water.
Japanese Patent Publication Kokai No. 7-157565 discloses a redispersible emulsion powder of a synthetic resin obtained by spray-drying an emulsion prepared by emulsion polymerization of ethylenically unsaturated monomer using an acetoacetylated (acetoacetyloxy group-containing) polyvinyl alcohol as a protective colloid for the purpose of improving the redispersibility and the water resistance of films formed therefrom. Japanese Patent Publications Kokai No. 9-151221 and Kokai No. 11-263848 disclose a synthetic resin powder wherein a polyvinyl alcohol polymer is chemically bonded to the surface of particles of a polymer of an ethylenically unsaturated monomer or a diene monomer through a terminal sulfide bond for the purpose of improving the redispersibility, the film forming property and the water resistance of films formed therefrom. Further, Japanese Patent Publication Kokai No. 11-279361 discloses an emulsion powder wherein a polyvinyl alcohol polymer having specific structural units such as carboxyethyl group is used for the same purpose as the above.
However, according to the present inventors"" investigation, the redispersible synthetic resin emulsion powder disclosed in Japanese Patent Publication Kokai No. 7-157565 has improved redispersibility and water resistance of films, but the improvement is not satisfactory. Further, in case of using vinyl ester monomers as the ethylenically unsaturated monomer, the obtained redispersible powder is poor in alkali resistance. In case of other ethylenically unsaturated monomers, the alkali resistance is good, but the emulsion polymerization stability is very low and it is difficult to obtain good emulsions.
The synthetic resin powders disclosed in Japanese Patent Publications Kokai No. 9-151221, Kokai No. 11-263848 and Kokai No. 11-279361 have improved redispersibility, film forming property and blocking resistance, but the improvement is not satisfactory. The water resistance is also not satisfactory, and further improvement of these properties is needed.
Thus, it is an object of the present invention to provide a synthetic resin powder which is excellent particularly in properties such as redispersibility, film forming property, water resistance and blocking resistance and, in addition, which is prepared by an emulsion polymerization with an improved stability.
This and other objects of the present invention will become apparent from the description hereinafter.
It has been found that a synthetic resin powder obtained from a synthetic resin emulsion containing a polyvinyl alcohol resin having an acetoacetoxy group and/or a mercapto group and having a block character [xcex7] of 0.3 to 0.6 meets the above-mentioned requirements.
Thus, in accordance with the present invention, there is provided a redispersible synthetic resin powder comprising particles of a polymer made from at least one member selected from the group consisting of ethylenically unsaturated monomers and diene monomers, and a polyvinyl alcohol resin having at least one member selected from the group consisting of acetoacetoxy group and mercapto group and having a block character [xcex7] of 0.3 to 0.6, wherein said polyvinyl alcohol resin is adsorbed onto the surface of said particles.
The term xe2x80x9cblock characterxe2x80x9d as used herein denotes a value obtained from the intensity ratio of peaks based on methylene carbon portions found within the range of 40 to 49 ppm in measurement of 13C-NMR using 3-(trimethylsiliy)propionic-2,2,3,3-d4acid sodium salt as an internal standard material [chemical shift of (OH, OH) dyad=46-49 ppm, chemical shift of (OH, OR) dyad=43.5-45.5 ppm, and chemical shift of (OR, OR) dyad=40-43 ppm, wherein OR denotes O-acetyl group and/or O-acetoacetyl group], and it is calculated by the following equation (1):
[xcex7]=(OH, OR)/2(OH)(OR) xe2x80x83xe2x80x83(1) 
wherein (OH, OR), (OH) and (OR) are calculated by molar fraction respectively, (OH) denotes a degree of hydrolysis (molar fraction) calculated from integral ratio in 13C-NMR and (OR) denotes the molar fraction of acetyloxy group and/or acetoacetyloxy group.